Proppants are solids, such as sand, ceramic, polymer, or composite particles, that are often used during fracture stimulation to keep a fracture open by resisting the closure stress applied by the geological formation above the fracture.
In many situations, a substantial portion of the proppant does not remain in a fracture where it has been placed but instead flows back to the wellbore, so that it is valuable to be able to assess the extent of any flowback. Furthermore, a knowledge of the locations of the proppant particles can also provide valuable information about the fracture geometry. The ability to monitor the locations of the proppant particles over time after their placement in a downhole environment is, therefore, a highly desirable objective. Progress towards the attainment of this objective has hitherto been both difficult to make and limited in its scope.
The patent to Ayoub et al. (U.S. Pat. No. 7,082,993), assigned to Schlumberger Technology Corporation, provides for a “Means and Method for Assessing the Geometry of a Subterranean Fracture During or After a Hydraulic Fracturing Treatment”. Disclosed therein is a method for determining the geometry of a hydraulic fracture where said geometry can be inferred from the use of a mixed proppant composition comprised of ferrous/magnetic fibers embedded in said proppant. The contrast in magnetic fields between the borehole environment and the surroundings can give the operator an indication of the fracture internals.
The patent application publication to Knobloch (U.S. 20060037755) provides for a “Solid State Pump”. Disclosed therein is a proppant containing a magnetostrictive material that is moved within an internal formation such as a geological reservoir of hydrocarbon through a solid state pumping action brought about by the introduction of a magnetic source.
The patent application publication to McCarthy et al. (US 20060102345 A1) describes a “Method of Estimating Fracture Geometry, Compositions and Articles Used for the Same”. This method involves mapping a subterranean fracture by using metallic particles with a certain dielectric constant that are contained within a proppant, and pulsing said particles with a particular electromagnetic radiation to perform said mapping.
The patent application publication to Entov et al. (U.S. 20070256830), assigned to Schlumberger Technology Corporation, provides for a “Method and an Apparatus for Evaluating a Geometry of a Hydraulic Fracture in a Rock Formation”. Disclosed therein is the use of an electrolyte-based proppant that, when injected into a borehole in a hydraulic fracturing event, causes an electrokinetic effect from the flow of said proppant through the borehole aperture. The geometry of the hydraulic fracture is mapped out via the detection of electric and/or magnetic fields triggered by the aforementioned electrokinetic phenomenon.
The following two books are recommended to readers who may be interested in general background information on piezoelectric and/or magnetostrictive materials: APC International, Ltd., “Piezoelectric Ceramics: Principles and Applications” (2002); and G. Engdahl (editor), “Handbook of Giant Magnetostrictive Materials”, Academic Press, New York (2000).